Method for reducing embrittlement condition of metals

ABSTRACT

A METHOD FOR REDUCING THE CONDITION OF EMBRITTLEMENT IN A METAL SPECIMEN BY IRRADIATING THE SPECIMEN WITH A NEUTRON FLUX OF SELECTED DENSITY. THE METHOD CONSISTS OF PLACING A METAL SPECIMEN IN VIEW OF NEUTRON RADIATION FOR A PRE-DETERMINED PERIOD OF TIME IN ORDER TO ALLEVIATE ANY EMBRITTLEMENT CONDITION AND/OR TO REDUCE LIKELIHOOD OF SUCH CONDITION AT A LATTER TIME, SUCH NEUTRON IRRADIATION TENDING TO REARRANGE THE INTERSTITIAL MAKEUP OF THE METAL SPECIMEN, E.G. BY BRINGING ABOUT DISSOCIATION OF DIATOMIC OR MOLECULAR HYDROGEN TO ITS MONATOMIC STATE.   D R A W I N G

April 16, v1974 RICHARD ETAL 3,804,734

METHOD FOR REDUCING EMBRITTLEMENT CONDITION OF METALS Filed April 19,1972 FUNCT/O/VAL SEPV/CE swm M M5 r H Y N m m M E MW WW Mm. TN A QUnited States PatentO 3,804,734 METHOD FOR REDUCING EMBRITTLENIENTCONDITION OF METALS Gordon L. Richard, 217 W. Coe, and Robert W.Blosser, 3620 Oak Grove Drive, both of Midwest City, Okla. 73110Continuation-impart of abandoned application Ser. No. 844,579, July 24,1969. This application April 19, 1972, Ser. No. 245,643

Int. Cl. B011 N US. Cl. 204157.1 H 12 Claims ABSTRACT OF THE DISCLOSURECROSS-REFERENCE TO RELATED APPLICATION The present invention constitutescontinuation-impart subject matter related to co-pending patentapplication Ser. No. 844,579 entitled Method for Reducing EmbrittlementCondition of Metals as filed on July 24, 1969 and now abandoned, in thename of the present inventors.

BACKGROUND OF THE PRESENT INVENTION 1) Field of the invention Theinvention relates generally to the treatment of metals for alleviationof weakening condition and, more particularly, but not by way oflimitation, it relates to a method for combating embrittlement conditionof metals. I

(2) Description of the prior art The prior attempts at alleviation ofmetal embrittlement conditions have been largely confined to varioustypes of heat treatment. That is, an approach wherein increased heat isapplied to the metal specimen in an attempt to drive hydrogen out of itssubstance. Such removal of hydrogen was effected through dissociationand diffusion as a result of baking of the metal specimen for apredetermined time at a selected temperature, and such procedure wasnecessarily time consuming. The prior art heat treating or bakingprocesses required baking of a specimen at the highest possibletemperature, thereby to reduce the time required as much as possible,but various temperature limitations were to be observed in order not todamage prior instilled desirable heat treatment properties. Aside fromthe prior types of heat application processes, the only other avenuesappear to be various attempts at complete avoidance of the hydrogen orother embrittlement condition in the first place. This may cause severelimitation in availability of treating processes, or it may necessitatean initial barrier coating deposition.

SUMMARY OF THE INVENTION The present invention contemplates a method oftreating metals to remove embrittlement conditions. In a more limitedaspect, the invention consists of irradiating a metal specimen inaneutron flux of pre-determined density in order to remove or alter theatomic structure of the embrittlement-causing hydrogen from thesubstance of the metal specimen.

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Therefore, it is an object of the present invention to provide a methodfor reducing probability of embrittlement failure of a metal morequickly than has been possible in prior known processing systems.

It is also an object of the invention to provide an embrittlementcombatting method capable of reducing production costs surroundingmanufacture, maintenance or re-conditioning of selected metal objects.

Finally, it is an object of the present invention to provide a methodfor the alleviation of hydrogen embrittlement in nondestructive mannerutilizing neutron radiation capable of permeating the physical structureof the metal to remove or alter embrittlement-causing molecules and/orions.

Other objects and advantages of the invention will be evident from thefollowing detailed description when read in conjunction with theaccompanying drawing which illustrates the invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a functional diagramillustrating various steps in the method of the invention; and

FIG. 2 is a perspective view, with parts shown in cutaway, of one formof apparatus which is suitable for carrying out the method of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The condition ofembrittlement of metals, particularly hydrogen embrittlement in highperformance, high reliability metal structures, has today become aproblem of some magnitude in certain areas of industry and technology.The condition of embrittlement is one, which highly increases theprobability of stress failures in metal structures, but which is quitedifiicult of detection at any time prior to the actual failure, i.e.cracking, flaking or such of the metal structure. Such a conditionbecomes a particular hazard in any industry or endeavor wherein highreliability must be imposed upon metals making up essential structures,e.g. aircraft industry, pipeline industry, the shipping and buildingindustries, to name only a few specific areas. Treatment with priormeans generally consisting of high heat treating may be so timeconsuming as to be prohibitive in many applications. Further, hydrogenremoval from such as titanium is impractical since it requires vacuumannealing at extremely high temperatures.

It is now known that certain treatment processes serve to hasten orbring about embrittlement conditions such that some form ofembrittlement alleviating treatment is necessary in order to enable theinitial metal processes. Thus, for example, hydrogen may be introducedduring melting and entrapped during solidification of metals, or it maybe picked up during heat treatment, electroplating, acid pickling,welding, and other processes as performed on metal specimens. Some formof embrittlement alleviating process becomes a necessity as a followupto certain of the above treatments, especially as employed in thefabrication or reconditioning of high reliability metal structures. Thepresent invention, as will be further described, is capable of rapidtreatment to alleviate the embrittlement condition with minimal increasein production time costs, etc.

A commonly held concept of hydrogen embrittlement is that monatomichydrogen precipitates at internal voids as molecular or diatomichydrogen. Such voids may be true voids, microcracks, or they may simplybe regions of high dislocation density. While the above is merely abasic idea of hydrogen embrittlement, it should be understood that thereare many more facets of the phenomena to be considered in gainingcomplete information as to the stress and fracture occurrences; however,the basic physical characteristics of the phenomena are ascertained to asufficient extent to support a more or less general definition of thepresent method. While all of the nuclear and interstitial events of theprocess would be difficult of explanation, the basic interaction ofmatter may be set forth in well-known terms and relationships.

A severe embrittlement condition can be produced in various metals bythe presence of but very small amounts of hydrogen. Finding support isthe fact that those metal structures of the body-centered cubic type andthe hexagonal close structured types are the most susceptible tohydrogen embrittlement. As little as .0001 weight percent of hydrogencan cause cracking or, at least, great reduction in ductility of steel.Hydrogen, being a small atom, is present in solution as monoatomichydrogen, and is capable of diffusing voids and interstitials of a metalstructure very rapidly, especially at increased temperatures. The mostcommonly held concept is that the monatomic hydrogen precipitates at theinternal void positions as molecular or diatomic hydrogen to bring aboutthe hydrogen embrittlement. Thus, when critical hydrogen concentrationis obtained, a crack or weak point of the metal arises. Suchembrittlement condition may show up as either cracks or flakes of themetal, depending upon the basic physical structure of the metal itselfat the point of embrittlement.

It appears that all ferritic steel may be subject to hydrogenembrittlement to some degree, and, generally, the variations in degreecan be correlated to the composition and micro-structure of theparticular steel. In addition, various other metals and metal alloys maybe subject to diffusive hydrogen action and embrittlement in varyingdegree, and embrittlement alleviation will often be desirable relativeto many metal working operations. The hydrogen embrittlement may befound in such as iron, iron alloys, copper, aluminum, titanium, andstill other metals and alloys of metals too numerous to itemize.

In accordance with the present invention, a metal specimen can beirradiated with a suitable intensity of neutron flux to remove orgreatly reduce the damaging hydrogen content of the specimen, and thisirradiation will not appreciably alter any other mechanical or physicalproperties of the metal specimen. Thus, as shown in FIG. 1, metalstructure having a possible embrittlement condition may be derived fromany of a specimen preprocessing 10, specimen cleaning 12, or otherspecimen handling stage for introduction through a neutron irradiationstep 16. After neutron irradiation of suflicient time and intensity, asdetermined in accordance with the size and shape of the specimen andother statistics pertaining to severity of embrittlement, this specimencan then be passed on or replaced in functional service step 18.

To take merely one example, for purposes of illustration, an aircraftlanding gear component may be passed through specimen cleaning 12 whichincludes paint removal through chemical means, plating removal throughelectrolytic means, pickling and replating, any of such processes beingknown contributors to the embrittlement condition; and, thereafter, themetal part is passed through the neutron irradiation step 16 for apre-determined time before return to functional service step 18, i.e.reassembly into the particular landing gear structure.

The neutron has special properties which make it particularly suitablefor such irradiation usage. The neutron has essentially the same mass asa proton but it has no electric charge, and this precludes accelerationof deflection by electric or magnetic fields as well as inhibition bysuch as a coulomb barrier. Thus, even the very slow neutrons can moveabout within electron shells of atoms since they have but very smallinteraction with electrons present therein.

Due to the operation of the above properties, the tightest structure ofmatter, in this instance, metal solids, is quite open to the neutron.Even the more crystalline solids with densely packed and tightly boundatoms are quite easily penetrated to relatively great depths by theneutron. When a neutron passes through matter such as this, it interactswith native nuclei of the matter in one of two dominant manners; thatis, neutron radiative capture (n, 'y) or elastic collisions between theneutron and the nuclei of the matter. For most structural metals withwhich the process is concerned, the radiative capture cross-section isso small that the primary interaction can be relied upon as that of theneutrons interacting with the nuclei of the matter.

In such elastic collisions, the greatest transfer of kinetic energy fromone to the other particle occurs when the two particles have equalmasses. Therefore, since protons and neutrons have nearly equal masses,the neutron can transfer a large percentage of energy to the proton uponcollision. In contrast to this, a neutron upon colliding with a heavynucleus such as that of most structural metals can transfer only a verysmall percentage of its energy to the heavy nucleus. The neutron willthen rebound from such a collision, retaining most of its originalenergy, then to proceed along a new path until another collision occurs.Such a process may recur repeatedly before the neutron losses as asignificant portion of its initial kinetic energy.

When hydrogen is present within the interstitial spaces of the specimenmatter, collisions between a bombarding neutron and a hydrogen nucleus(proton) are possible. Such collisions can supply the necessary energywhich is required to separate diatomic hydrogen into two free protons,while also imparting significant kinetic energy to these protons. Fordiatomic hydrogen, the dissociation energy is equal to 4.48 electronvolts. The binding energy of an electron and a proton forming a hydrogenatom has been shown experimentally to be equal to 13.58 electron volts.Thus, the binding energy, and the dissociation energy (the total energyrequired to separate diatomic hydrogen into two free protons of hydrogennuclei), is equal to 18.06 electron volts.

Elastic collisions between a bombarding neutron and a hydrogen nucleusor proton can provide the necessary 18.06 electron volts which arerequired to separate the diatomic hydrogen into two free protons. Thus,when a neutron flux impinges upon a solid, such as a selected metallicspecimen, the majority of the neutrons will enter the metal specimen andprogress through the solid such that their net displacement is away fromthe neutron source. Hydrogen atoms which are entrapped within the solidwill experience neutron collisions, and in those collisions wherein theenergy transfer is high enough, the atoms will be ionized and caused tomove in the general direction of the neutron flux.

A continuation of the irradiation for a sufficient period of time willcause most of the hydrogen in a metal specimen to migrate out of thesolid along the direction of the neutron flux radiation. The timerequired to reach a desired degree of freedom from the hydrogen willdepend upon the fiux density and energy spectrum. 'It will also dependupon the physical properties of the metal specimen for which thehydrogen is being removed. The quantity of hydrogen Q remaining in agiven item after a process time t'will take the following form:

wherein the constant k is determined by the neutron flux density andenergy spectrum as considered with the mechanical and physicalproperties of the particular metal specimen, and Q, is equal to theoriginal quantity of hydrogen present in the specimen.

The method of the invention may be carried out in such as an apparatus20 which is shown in FIG. 2. The apparatus 20 may consist of anysuitable enclosure such as rectangular enclosure 22 preferably having anaccess or door 24 provided therein. Enclosure 22 may be formed from arigid structural material which is internally lined by a suitableneutron absorptive shielding layer 26, e.g.

polyethylene or other shielding material which exhibits the desiredimpervious quality to neutron radiation.

A neutron source 28 may be positioned within enclosure 22 in position toemit radiation 30 for continuous coverage over an area 32 on asupporting base 34. A shielding panel 36, e.g. a polyethylene or otherneutron absorptive slab may be supported across the closure 22 to definean aperture 38 through which radiation 30 from source 28 is directedtowards area 32.A sliding shutter 40, also constructure of neutronabsorptive material, is positioned for manipulation to close up aperture28 during times when a metal specimen is being changed, turned, .orotherwise moved in a lower irradiation chamber 42.

The supporting base 34 may beany suitable structure upon which the metalspecimen may be rested or otherwise secured, and such base 34 may bemade slidable for access through door 24, e.g. slidable along such as apair of rails 44 and 46 in coaction with slide ways or grooves 48 and50, respectively. It should be understood that the irradiation chamber42 may include any of various mechanisms which facilitate insertion andremoval, as well as periodic turning or moving, of the metal specimenprior to or during the irradiation periods. It may also contain ahydrogen free purge gas such as nitrogen. The general 1 size and shapeof the specimen undergoing irradiation will bear greatly upon the sizeand type of facility employed as the shielded neutron irradiationenclosure and its supporting members. p

An electrostatic field may be employed to preclude reabsorption ofhydrogen ions (protons) which have been driven out of the metalspecimen, and hydrogen ions which have been generated due to the neutronflux acting upon any hydrogen which may be present in the atmospheresurrounding the specimen. This electrostatic field may be produced bycharging the specimen to a high positive potential relative to theenclosure or to a suitable cathode within the enclosure.

The neutron source 28 may be any of various commercially availableneutron sources which emit the desired density of radiation for theparticular application. For low intensity usage, it may be sufiicient toemploy a wellknown form of radium-beryllium, 'americium-beryllium, etc.source. For still other applications requiring very high energy neutronson the order of 2 bev., or other energy source of commercially availabletype. Still other sources are available such as photo-distintegrationreaction', nuclear reactions through accelerated charged particlecollisions, a deuteron stripping reaction, etc.

The method of the invention was carried out in a series of testprocedures wherein metal specimens of a standardized proof test typewere subjected to induced brittlement condition and then subjected tothe present method to bring about de-embrittlement. The metallic testspecimens are a commercially available test strip known as the HEP(Hydrogen Embrittlement Proof) strips, as manufactured by the ChemicalCorporation, 57 Walthan'1 Ave., Springfield, Mass. I v p The HEP stripsare comprised of a metal alloy which has a very high sensitivity tohydrogen embrittlement. The strips are normally used as an indicatorduring such as a plating operation to determine the extent of hydrogenembrittlement which might be incurred by each vat of items being plated.Thus, the HEP strips are processed along with the items being plated,and are then loaded into a test fixture which produces a static bendingload. The time to break is then an indication of the amount of hydrogenabsorbed by the item during the plating operation. Further informationas to the metallic composition of the 'HEP strips and the particularstatic loading fixture are available with reference to US. Pat. No.3,455,152.

The pilot testing of the present invention consisted of using fifty HEPstrips in ten different testing procedures of five strips each. Thetesting consisted of the following steps:

(a) Pre-cleaning the HEP strips-The test strips were de-greased inacetone and then scrubbed in a detergent solution. The strips were thenwashed in running tap water, air dried and again washed in acetone.

(b) Cathodic charging-The HEP strips, those which were to be embrittledwith hydrogen, were placed in a 0.75 normal sodium hydroxide solutionand cathodically charged at 16 amperes per square foot for a specifiedtime period, i.e. 1.5 hours.

(c) Neutron irradiation.--The HEP strips, those which were to receiveneutron irradiation, were irradiated in a gas tight enclosure which wascontinuously purged with dry atmosphere. The dry atmosphere purge gaswas produced by passing shop air through activated silica gel and theninto the irradiation enclosure. A flow rate of approximately twostandard cubic feet per minute was maintained during the irradiationprocess. I

The specimens or HEP strips were mounted upon a rack within theenclosure such that they approximately formed a segment of the surfaceof a cylinder with a .12" diameter. The neutron source was then locatedat the midpoint of the axis of the defined cylinder. The particular formof neutron source used was a radium-beryllium sealed source asmanufactured by Monsanto Inc. This type of source is normally used in aTroxler Electronic Laboratories, Inc. Model'104 moisture gauge.

The particular source utilized was manufactured in October 1965, and atthat time had a specified neutron output of 443x10 neutrons per second.Since the specified half-life for this source is 1,620 years, it isreasonable to assume that this activity is not appreciably diminished atthis time. The neutron flux produced by this source has a sphericalpattern, thus a one-inch square on the surface of a 12" diameter spherewith the source at its center would have a neutron flux of approximately98 neutrons per second. This will very closely approximate the fluximpinging upon the test specimens as they were mounted upon the holdingrack within the irradiation enclosure.

Static load testing of the individual HEP strips was carried oututilizing a loading test fixture as is the subject of theabove-mentioned US. Pat. No. 3,455,152. The static load testing machineis a commercially available de- 'vice" which is'utilized for derivingembrittlement indication in various segments of industry. HEP strips tobe tested are placed in'the fixture such that they are held in abowed orU-shape with the tips 3.5 inches apart. Pressure is applied only at theends of the strips, in accordance with manufacturers recommendation forvarious applications. The HEP strips are of a standardized size 7 /2"long, wide, and of varying thicknesses depending upon the exigencies ofapplication, a .020" thickness being utilized in the presentapplication.

The HEP strips were processed in batches of five strips, each batchbeing subjected to significantly different test parameters. A total offifty strips or ten batches of five strips were utilized, and allbatches were pre-processed through the same pre-cleaning operation(aforedescribed) immediately before cathodic charging.

TEST NO. 1

Five test strips were pre-cleaned and loaded directly into the staticload test fixture without cathodic charging or neutron irradiation.These specimens were left under static load for 336 hours, during whichtime none failed.

TEST NO. 2

7 TEST NO. 3

The test strips were pre-cleaned, cathodically charged at 16 amperes persquare foot for 1.5 hours, re-cleaned in running water and forced airdried, and subjected to neutron irradiation for one hour. A firstspecimen failed after 72 minutes, a second specimen failed after 83minutes, a third specimen failed after 87 minutes, a fourth specimenfailed after 105 minutes, and the fifth specimen failed after 117minutes.

TEST NO. 4

Test strips for test No. 4 were subjected to the same initial proceduresof pre-cleaning and cathodic charging except that they were subjected toneutron irradiation for two hours. Static load testing of these fivespecimens resulted in the following:

The first specimen failed after 114 minutes; the second specimen failedafter 132 minutes; the third specimen failed after 157 minutes; thefourth specimen failed after 205 minutes; and the fifth specimen wasleft in the static loading fixture for 72 hours, but did not fail.

TEST NO. 5

The five test strips were subjected to the same pre-cleaning andcathodic charging except that they were subjected to neutron irradiationfor a period of four hours. The times of failure are as follows:

First specimen-5 .25 hours; second specimen-5.5 hours; third specimen-7.5 hours; fourth specimen12 hours; and fifth specimen18 hours.

TEST NO. 6

The test strips were similarly pre-cleaned and cathodically charged toreceive neutron irradiation for a period of eight hours. The times offailure during static load testing were as follows:

Specimen No. 1-6 hours; and the remaining specimens 2 through 5 wereleft in the loading fixture for 168 hours during which time no morefailures occurred.

TEST NO. 7

The test strips were subjected to the same pre-cleaning and cathodiccharging plus neutron irradiation for a period of 12 hours. Each of thefive test strips were left in the static load fixture for a period oftwo weeks, during which time there were no failures.

TEST NO. 8

The test strips were subjected to the same pre-cleaning and cathodiccharging plus neutron irradiation for a period of 16 hours. Each of thefive test strips were left in the static load fixture for a period oftwo weeks, during which time there were no failures.

TEST NO. 9

The test strips were subjected to the same pre-cleaning and cathodiccharging plus neutron irradiation for a period of 24 hours. Each of thefive test strips were left in the static load fixture for a period oftwo weeks, during which time there were no failures.

TEST NO. 10

The test strips were subjected to the same pre-cleaning and cathodiccharging plus neutron irradiation for a period of 48 hours. Each of thefive test strips were left in the static load fixture for a period oftwo weeks, during which tion exhibited no failures for a period of 336hours and would have continued under test by an unknown time. However,test No. 2 received induced embrittlement by cathodic charging and thespecimens failed under static load testing in a relatively short time onthe order of minutes.

Inspection of the data for tests Nos. 3 through 10 then indicates theincreasing resistance to breakage under static load test in proportionto time of irradiation at a predetermined neutron flux rate; and resultsof tests Nos. 7 through 10 indicate that the metal test strips werecompletely restored to their high embrittlement resistance constituency.The test results show conclusively that the method of the inventionserves to alleviate hydrogen embrittlement condition in metals torestore the metal specimens to their original physical characteristicsenabling requisite stability and reliability.

The foregoing discloses a novel method for eliminating or alleviatingthe condition of embrittlement in certain metals. In particular, themethod succeeds in enabling rapid reduction of embrittling hydrogenpresence within the structure of metal specimens. Such a neutronirradiation method as disclosed herein offers a vastly more economicalprocess wherein hydrogen embrittlement caused by metal treatingconditions or processes can be eliminated or significantly reduced i.e.to an acceptable level.

Changes may be made in the combination and arrangement of steps asheretofore set forth in the specification and shown in the drawing; itbeing understood that changes may be made in the embodiments disclosedwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A method for reducing hydrogen embrittlement condition of anembrittled metal specimen comprising the step of:

irradiating the metal specimen with neutrons thereby increasing theratio of monatomic hydrogen atoms to diatomic hydrogen molecules andalleviating the condition of embrittlement. 2. The method of claim 1wherein said metal is ferritic steel.

3. The method of claim 1 wherein said metal is iron.

4. The method of claim 1 wherein said metal is a carbon-ironcomposition.

5. The method of claim 1 wherein said metal is copper.

6. A method as set forth in claim 1 wherein the metal is aluminum.

a 7. The method of claim 1 wherein the metal is an alloy of iron oraluminum.

8. The method of claim 1 wherein said metal is titanium.

9. The method of claim 1 which is further characterized in that:

said step of irradiation is effected by positioning the metal specimenfor impingement and permeation of neutrons within a neutron flux.

10. The method as set forth in claim 9 which is further characterized inthat:

said neutron flux is generated within a shielding enclosure which isimpervious to neutrons.

11. The method as set forth in claim 9 wherein the metal is an alloy ofcopper.

12. The method of claim 1 which is further characterized in that:

said metal is charged to a positive potential, producing anelectrostatic field about the metal for accelerating hydrogen ions awayfrom the metal.

References Cited UNITED STATES PATENTS 3,310,395 3/1967 Swartz et al.148-133 3,440,037 4/1969 Martin et al. 75128 3,496,034 2/1970 Alger etal. 148-36 HOWARD C. WILLIAMS, Primary Examiner

